This invention relates to a piston-cylinder unit. More particularly, this invention relates to a piston-cylinder unit for use in a disc brake assembly. The piston-cylinder units of the invention are particularly well suited to be used in disc brake assemblies for automobiles.
Throughout the specification, numerous references will be made to the use of the piston-cylinder unit in an automotive disc brake assembly. However, it should be realized that the invention could be used in other types of piston-cylinder unit assemblies.
As well known in the art, automobile disc brake assemblies include such types as fixed caliper and floating caliper. In the fixed caliper type, a caliper member is secured to the vehicle body and is located adjacent to a peripheral portion of the brake disc secured to a rotating wheel axle. The caliper member has a piston-cylinder unit mounted in the inner side of each of its two arm sections. Each piston has an end, adjacent to a side of the brake disc, which is provided with a brake pad. The pad is generally forced against the side of the brake disc during braking action when hydraulic pressure from a master cylinder actuates the piston.
In a disc brake assembly of the floating caliper type, a caliper member is slidably attached to the vehicle body. The caliper member has a piston-cylinder unit mounted on the inner side of one of its two arm sections. The piston has an end, adjacent to one side of the brake disc, which is provided with a brake pad. The brake pad is generally forced against the side of the brake disc during braking action when hydraulic pressure from a master cylinder actuates the piston. A reaction force of the brake pad located on the piston being forced against the brake disc causes a movement of the caliper member in a direction opposite to the axial movement of the piston. This reaction force causes a brake pad which is attached to the inner side of the other arm section of the caliper member to be simultaneously pressed against the other side of the brake disc so that a complete braking action is effected.
In the conventional piston-cylinder unit used in the aforementioned disc brake assemblies, it is necessary to provide a seal between the piston and the cylinder which is subjected to fluid pressure during hydraulic actuation of the piston. The seal is generally accomplished by a seal ring fitted into an annular groove formed on the inner peripheral surface of the cylinder or the outer circular wall of the piston. The seal ring is urged against either the piston wall or the cylinder surface with an initial radial stress or strain produced in it. Due to this radial stress, a frictional force exists between the seal ring and the surface of the piston or cylinder in contact with the seal ring at the time of axial movement of the piston in the cylinder. Because of this frictional force, some of the braking force of the brake pad against the brake disc is lost.
In addition to this braking force loss, a frequent problem has been encountered in that the hydraulically actuated piston will not return to its initial axial position when the fluid pressure on the piston is removed. Consequently, the brake pad carried by the piston may remain in contact with the rotating brake disc so as to create an unwanted "dragging" effect. On the other hand, if the piston is brought to an axial position beyond its initial axial position during the return stroke, a shortage of the operating fluid pressure could result in a subsequent braking operation. This shortage would occur because of the increased stroke of the piston necessary to attain a braking action.
Accordingly, it is a primary object of this invention to provide a new and improved piston-cylinder unit having a flexible seal member so as to improve the action force of the piston by eliminating friction loss from a sealing member between the piston and the cylinder.
A still further object of this invention is to provide a new and improved piston-cylinder unit which can be used in automotive disc brake assemblies to eliminate the above-described problems.
Another object of this invention is to provide a new and improved piston-cylinder unit for use in a disc brake assembly that will prevent dragging of the brake pad against the rotating brake disc.
Still another object of this invention is to provide a new and improved piston-cylinder unit for use in a disc brake assembly that will keep the piston from being brought to a return axial position greater than its initial axial position and thus prevent a shortage of operating fluid pressure from occurring in a subsequent braking operation.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the piston-cylinder unit of this invention comprises (1) a cylinder; (2) a hydraulically actuated piston received in the cylinder for axial movement therein; and (3) a flexible seal member having a first end attached in sealing engagement to the head of the piston and a second end attached in sealing engagement to the cylinder axially opposed to the piston head for elastic expansion and contraction in an axial direction, the flexible seal member defining between the piston head and the cylinder a chamber for receiving fluid pressure for actuation of the piston.
Preferably, the flexible seal member comprises a bellows. It is also preferred that the piston-cylinder unit include flexible means interconnecting the circular wall of the piston and the opposing wall of the cylinder for preventing particles such as dust and water droplets from entering between the piston and the cylinder.
The invention consists in the novel parts, constructions, arrangements, combinations and improvements shown and described. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention.